The present invention relates to a method for correcting the zero pressure value of a hydraulic power steering pressure sensor, to permit accurate detection of a working pressure of a hydraulic power steering unit.
As disclosed in Japanese Utility Model Provisional Publication No. H2-85675 (U.S. Pat. No. 5,018,594, DE3942494, GB2228460), a method for detecting the neutral point of a steering wheel by utilizing a working pressure of a hydraulic power steering unit is conventionally known. The present inventor has already proposed, under Application Number H3-325888, a method for estimating the neutral point of a steering wheel by taking a car speed and a steering wheel angle into account in addition to the working pressure, i.e., the power steering pressure of the hydraulic power steering unit.
Such a method is based on the assumption that the working pressure of the power steering unit is accurately detected. However, it should be noted that the output characteristics of pressure sensors, which detect the power steering pressure, may vary among different pressure sensors and may also vary due to changes in the environments wherein the sensors are used, or may vary due to other factors. In such a case, a discrepancy occurs between an actual value on the pressure sensor and the rated value. Further, the sensor value of the pressure sensor fails to indicate a zero pressure value even when the hydraulic power steering is not in operation.
Under such a circumstance, the neutral point of the steering wheel, which is estimated according to the aforementioned method, is inaccurate. This leads to a deficiency in that the tracing control of a traction control system or the rear-wheel steering control based on the estimated neutral point cannot be performed with high accuracy.